(57) An interpolator or decimator filter structure (Fig. 3) operable between two sampling
rates (Fs and 2Fs), and in which two branches (21 and 23) of the structure are divided into low and
high frequency sections by sampling switches (27 and 29). The high frequency section
of one branch (21) incorporates a composite filter comprised of a nesting filter (1')
and a nested filter (1). Each of these filters (1', 1 ) includes a digital coefficient
multiplier (5', 5). The coefficient (K2) of the coefficient multiplier (5), part of the nested filter (1), is preset to a
value such that 3dB attenuation occurs at a point offset in frequency from one-half
the lower sampling rate (Fs).
The composite filter (1'), is characterised by a transform function B(2) given by
the expression:
where A(Z) is the transform function of the nested filter 1:
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